import pandas as pd
import numpy as np

b = pd.Series([1,2,3,4],index = ['a','b','c','d'])
d = pd.Series([1,2,3,4,8],index = ['c','d','a','b', 'e'])

e = b + d
# print(e) 

print(e.isnull())
print(e.notnull())
print(e.dropna())
print(e.fillna(0))
print(e.fillna(e.mean()))
